In the quest for imaging systems that are very small and flexible, yet don't require elaborate protective cases, a team of researchers at MIT Media Lab have scaled things down with a lensless imaging device called a "optical brush." The device uses a loose bundle of optical fibers to produce images that could lead to more compact and robust ways to study oil fields and build smaller endoscopes.
The optical brush created by the team consists of an array of photosensors at one end connected to a bundle of 1,100 optical fibers, which are left to wave free to move through micrometer-scale gaps in a porous membrane. As they move, these loose ends collect light and transmit it to an array.
Unlike in more conventional fiberoptic systems, the loose end of the bundles don't need to line up with the array and can wave where they will. Normally, this would just produce a scattered blur, but the MIT scientists used a technique called "time of flight" to create order out of apparent chaos. This involves shining a burst of light on the target and measuring the difference between the burst and when the light reaches the array. It allows the system to use an algorithm to calculate the fiber's position and build up an accurate image.
In the case of the MIT experimental setup, the bursts were sent from two separate lasers shining perpendicularly on the target, but in a practical version the fibers themselves would supply the light bursts. For test purposes, the fibers were focused on a series of patterns with the end of the fiber bundle running through a beam splitter feeding into a regular camera and a high-speed imager for measuring the speed of the light bursts. This allowed the algorithms to create a two-dimensional map of the fiber tips and deduce the image.
Since the resolution was only 33 by 33 pixels thanks to the 300 micrometer fibers used, the test images were fairly blurry, but the team says that if more fibers with a finer diameter were used, the resolution would greatly improve without increasing the bundle size. In addition, by sending many pulses through the fibers to illuminate the object, calibration of images would take only a fraction of a second, and using interferometry, whereby the light waves are superimposed over one another, could also boost resolution.
According to the team, the technology could one day be used for robust imagers for use in surveying oil fields and aquifers, the inspection of plumbing systems, and for creating endoscopes with narrower diameters.
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